As a wireless microphone device for converting a voice signal from a microphone into a high frequency signal to wirelessly transmit it, handheld type device and two-piece type device are known. The handheld type wireless microphone device is a handheld wireless device in which microphone and transmitter units are integrated. The two-piece type microphone device is a wireless device in which a microphone unit and a transmitter unit are configured to respectively have separate housings, and connected to each other with a flexible transmission cable. The two-piece type microphone device is attachable to a waist belt, and therefore sometimes called a belt pack type device. Such two-piece type wireless microphone device has conventionally used a ¼λ whip antenna, a helical antenna, or a loop antenna as a transmitting antenna for the high frequency signal.
The ¼λ whip antenna is an antenna using as an antenna element a linear conductor having a length corresponding to a ¼λ of a transmitting radio wave, and used with being drawn out of the housing of the transmitter unit. The helical antenna is an antenna using a coil-like conductor as an antenna element, and characterized by a high Q factor (Quality factor) as compared with the ¼λ whip antenna. The loop antenna is an antenna using a loop-like conductor as an antenna element, and characterized by having an extremely high Q factor.
The wireless microphone device using the ¼λ whip antenna is used with the antenna being protruded from the housing of the transmitter unit, and therefore there arise problems that the transmission cable or a human body, and the antenna are likely to interfere with each other, and an antenna part is likely to be broken. Also, the antenna is used with being exposed, and therefore there arises a problem that radiation characteristics may be largely varied and sensitivity may be reduced due to a change in surrounding environment caused by the human body. On the other hand, in the wireless microphone device using the helical or the loop antenna, a frequency band for good radiation efficiency is narrow, and therefore there arises problems that a reduction in sensitivity upon variation of radiation characteristics due to change in surrounding environment is large, and that the antenna cannot be shared between wireless microphone devices respectively having different operating frequency bands.
In general, as the transmitting antenna for a high frequency signal, a ½λ dipole antenna is known in addition to the above described antennas. The ½λ dipole antenna is an antenna in which two linear antenna elements are arranged in their common longer direction, and end parts facing to each other are fed with a transmission signal. The ½λ dipole antenna is characterized by increasing a diameter of the antenna elements or using planate conductors as the antenna elements to thereby moderate the variation in radiation characteristics (e.g., variation in antenna impedance) due to a human body and widen a frequency band having good radiation efficiency.
The wireless microphone device using such a dipole antenna as a transmitting antenna is described in, for example, Patent document 1 or 2. The wireless microphone device described in Patent document 1 is a handheld type microphone device using a transmission cable for transmitting an electrical signal from a microphone to a circuit element on a circuit board and an electrical conductor in a housing as respective antenna elements of the dipole antenna.
The wireless microphone device described in Patent document 2 is a handheld type microphone device using circuit boards as antenna elements of the dipole antenna. The microphone device using the circuit boards as the antenna elements of the dipole antenna is advantageous in miniaturizing a housing and reducing manufacturing cost as compared with the case where the antenna elements are separately provided, and configured to have the planate antenna elements, and therefore the variation in radiation characteristics due to a human body can be moderated.
However, if a transmission circuit is provided on the circuit board used for the antenna element of the dipole antenna, a length of the circuit board, which effectively acts as the antenna element, is shortened, and therefore there arises a problem that desired radiation characteristics cannot be obtained.
FIGS. 9 (a) and (b) are diagrams illustrating an example of a configuration inside the conventional wireless microphone device, in which a dipole antenna 100 using two circuit boards 101 and 102 as the antenna elements respectively is illustrated. FIG. 9 (a) illustrates a front view of the dipole antenna 100 as viewed from a direction vertical to a face of the board, and FIG. 9 (b) illustrates a side view of the dipole antenna 100. In the dipole antenna 100, the circuit board 102 is provided thereon with an oscillator 103, and a high frequency signal generated by the oscillator 103 is fed to the respective circuit boards 101 and 102 through feeding points 105. The feeding points 105 are positioned on the other circuit board 101 side of the oscillator 103, and feeding paths 104 are provided from the oscillator 103 to the feeding points 105. On the feeding paths 104, an amplifier circuit for power-amplifying the high frequency signal, band-limiting filter for limiting a frequency band of the power-amplified high frequency signal, and the like are provided.
On the oscillator side 103 in the dipole antenna 100, a feeding direction of the high frequency signal is opposite between the feeding paths 104 and the circuit board 102, and therefore an overlap region A between the feeding paths 104 and the circuit board 102 does not effectively function as an antenna element due to electromagnetic coupling. For this reason, in a longer direction of the dipole antenna 100, a length of the circuit board 102, which effectively acts as the antenna element, is shortened, and therefore desired radiation characteristics cannot be obtained. In particular, if the circuit board 102 provided with the oscillator 103 is used as the antenna element of the dipole antenna, such phenomenon becomes significant, and therefore there arises a problem that in order to obtain the desired radiation characteristics, the circuit board 102 is increased in size, resulting in an increase in size of a housing of the microphone device.    Patent document 1: Japanese patent No. 3227142    Patent document 2: Japanese patent No. 3640744